I. Field of the Invention
The present invention relates to a self-piercing rivet of the kind that is inserted into sheet material without full penetration such that a deformed end of the rivet remains encapsulated by an upset annulus of the sheet material. It also relates to a method for forming a joint with such a rivet and to a rivet insertion system.
II. Description of Related Art
A self-piercing rivet generally has a head and a partially hollow shank. It is driven by a punch into the sheet material such that it pierces the top sheet and forms a mechanical interlock with the bottom sheet with the head often (but not always) flush with the upper surface of the top sheet. Since the bottom sheet is not pierced there is a reduced risk of corrosion occurring in the completed joint. Using self-piercing rivets in a joining process reduces the number of production steps as compared to conventional riveting in which a hole first has to be drilled into the sheet material before the rivet is inserted and then its projecting ends upset.
Self-piercing riveting has been used to great commercial success in the automobile industry where light-weight materials, such as aluminium, have been adopted for vehicle body panels in the interests of weight reduction and therefore reduced energy consumption. Aluminium is difficult or not feasible to spot weld, particularly to steel, owing to its high thermal conductivity, low melting range and propensity to form oxide surface film.
More recently in the automotive industry there has been a move towards using high strength sheet metals. Our European patent no. 2024651 describes a self-piercing rivet particularly suitable for joining high strength, thick stack steels. Since such steels have a high ultimate tensile strength the insertion forces applied to the rivet are necessarily high there is thus a significant risk of rivet collapse. The rivets must be treated to give them a medium/high hardness value (e.g. 450-510 Hv) so that they have sufficient strength. It has been established that such a rivet is not always suitable for use with thick stack, high strength light metal alloys such as magnesium and aluminium alloys especially where the material combination may have three or more layers. Moreover, other conventional rivets are not generally suitable for joining such materials.
Aluminium alloy sheet material generally exhibits superior ductility and so dies with relatively deep cavities tend to be used but the joints suffer from the tendency for the middle sheets to push through the lowermost sheet in the insertion process. This leaves a weakened joint which is often prone to corrosion and it may not be possible to produce a satisfactory joint repeatedly in a mass production environment.